Zhang et al: Angiogenic Gene Therapy for Improving Islet Graft Vascularization control, in comparison to control islets that are transplanted in the absence of VEGF protein. These results suggest that local VEGF delivery to islet grafts improves the outcome of islet transplantation by enhancing islet revascularization (Sigrist et al, 2002). To improve islet graft vascularization, we have delivered the human vascular endothelial growth factor (hVEGF) cDNA by adenoviral-gene transfer to mouse islets, followed by transplantation under the renal capsule in streptozotocin-induced diabetic mice (Zhang et al, 2003). We showed that all the renal capsules containing the hVEGF vector-transduced islets (250 islets) displayed significantly higher functional islet mass, as measured by insulin immunostaining, and greater vascular density, as determined by immunostaining of CD31, the platelet endothelial cell adhesion molecule-1 (PECAM-1) (Watanabe et al, 2000). As a result, diabetic mice receiving the hVEGF vector-treated islets exhibited normoglycemia with improved glucose tolerance. In contrast, diabetic mice receiving an equivalent islet mass that were pre-transduced with a control vector maintained moderate hyperglycemia with impaired glucose tolerance. These results provide the proof-of-principle that angiogenic gene transfer to islets prior to islet transplantation allows local production of VEGF in islet grafts, which in turn stimulates graft angiogenesis and augments islet revascularization (Zhang et al, 2003). While therapeutic angiogenesis, so called biobypass, has been considered an alternative modality for treating coronary and peripheral artery diseases, based on the efficacy and safety of plasmid- or adenoviral vectormediated VEGF delivery in angiogenesis in a number of preclinical studies and clinical trials (Isner, 2002; Koransky et al, 2002; Mercadier and Logeart, 2002; Rasmussen et al, 2002; Sylven, 2002, Khan et al, 2003; Kusumanto et al, 2003), our view is that a similar
angiogenic strategy should be explored to accelerate islet graft angiogenesis, allowing rapid and adequate islet revascularization post transplantation. Such an approach, when used in conjunction with islet transplantation, has the potential for improving the success rate and clinical outcome of islet transplantation with long-term glycemic control at a reduced cost of islets.
2. Ex vivo gene delivery to islets The rationale for enhancing islet graft vascularization by angiogenic gene transfer is as follows: islets are transduced in culture with a vector expressing angiogenic molecules, such as VEGF, followed by transplantation into a diabetic subject, as illustrated schematically in Figure 2. Using an adenoviral-mediated gene delivery system, we have validated this concept by showing that VEGF production in newly transplanted islets significantly improves islet revascularization and functional islet mass (Zhang et al, 2003). It is noteworthy that adenoviral vectors are associated with immunogenecity. In addition, islets are terminally differentiated post-mitotic cells, which poses a great challenge for ex vivo gene delivery to islets by vectors whose transduction depends on cell division (Ito and Kedes, 1997; Robbins and Ghivizzani, 1998). However, recent advances in both viral and nonviral vector development have made it feasible to transfer genes to intact islets ex vivo at reasonable efficiencies without adversely affecting the architecture and function of islets. Below is a focused review of a number of vector systems that are currently in use for ex vivo gene transfer to isolated islets.
Figure 2. Schematic representation of angiogenic gene transfer in conjunction with islet transplantation. Islets are isolated and incubated in culture media in the presence of a gene vector that expresses angiogenic molecules. After transduction, islets are transplanted intraportally into the liver of a diabetic subject.
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